Left tachometer stopped working. With weird diagnostic event

[Doug Overkill]

Here is a strange one, or at least is to me. 1997 Challenger 1800 with twin 787’s. Recently the left tachometer just stopped working. No needle movement at all. The right works fine. I was trying to determine if it’s the signal from the MPEM or the tach itself. In the console behind the dash each tach is plugged in with a 3 wire connector. Both connectors have Black and purple wires that read 12.8v when you wake up the MPEM by quick pressing the starter buttons. The 3rd wire is grey (1 with tracer).

Here’s where it gets weird. When I swap the 2 connectors and start both motors, the right gauge still reads the RPM from the right engine. The left tach still doesn’t move. Swap them back and right gauge reads right engine. Huh?

If the connector with tracer (right motor) is plugged into right tach it works as it should. Both when the left connector is plugged in or not. If I plug that right connector into the left gauge nothing happens if the left is unplugged. When I add the left motor connector to the right tach, then the right tach displays the right motor rpm. Again huh?

So confused. Anyone have this experience? Anyone have any ideas as to what to do? It is very helpful when driving this boat to have both engine tachometers working.

 

[Doug Overkill]

Ok so I messed up. There are 4 wires coming out of the tachometer on this boat. The 3 connector I was swapping is the 12v hot and neutral plus the dash light. There is also a single connector that has the grey signal wire. I wasn’t swapping this in my earlier tests. When I did the left motor shows on the right tach and nothing happens on left tach.

Oops. Careless diagnosis.

 

[Doug Overkill]

Now that I know left tach is broken and there is a piece rattling around inside. Now that it is out of the dash I can hear it. $150-$250 on eBay and I decided NOTHING TO LOSE.

Grab the cutoff wheel and cut it open. Inside I found the loose piece was a low voltage dropout regulator. There was only 1 place on the board that had 3 terminals. It had been on the other side of the board but I didn’t need to cut that much more of the gauge apart. A little hot glue to hold it in place. A little solder to connect it to the circuits. I tried it and it works! A bit more hot glue to make sure it stays put. And a bunch more hot glue to reassemble the housing.

Fingers crossed and it hopefully works for as long as the boat. Here are a bunch of pictures.